A major hindrance to providing reliable mail service is the lack ofwell-defined FTP error replies that would enable a mailing process todecide how to handle a failure. New FTP error codes are currently inthe design stage, and a proposal will be announced soon. In theinterim, we can get some improvement by simply defining how we intend touse the current FTP codes. The purpose of this RFC is to inform allsites of how TENEX sites will use and interpret the codes starting inthe near future.

450, 451, and 453 are applicable to both the MAIL and MLFL commands,while 452 and 454 are only meaningful for MLFL.

SHORTCOMING OF CURRENT DEFINITIONS

There are more possible causes of failure to deliver mail than theones defined above. Implementors of FTP servers thus had to makearbitrary assignments of error conditions to defined codes. As aresult, although the text of the reply might distinguish theseconditions for the benefit of human users, the code doesn't distinguishthem for the benefit of processes.

The minimum distinction needed by the TENEX mail-sending processesis between permanent and non-permanent failures. In the latter case,the process will repeatedly try to deliver the mail for several days.

Sussman [Page 1]

RFC 630 FTP Error Code Usage for Mail Service April 1974

NEW DEFINITIONS FOR TENEX USE

The following changes will be installed at TENEX sites over the nextcouple of months.

FTP SERVER

The TENEX FTP server will continue to use 452 and 454 as specifiedfor the MLFL command.

For MAIL and MLFL, it will send the other codes as follows:

450 Permanent failures due to the user addressed in the Mail or MLFL command.

Examples: No such user; No mailbox for that user; Can't access file (because net users can't write in that mailbox).